It is well known in the art to use suitably engineered and prepared diagnostic ophthalmic lenses to provide an indirect image of the retina of a patient under examination. By properly placing a diagnostic ophthalmic lens a suitable distance from the cornea of an eye, an indirect image of the retinal surface is formed external to the eye structure. The indirect image of the curved retinal surface is typically formed in a generally flat image plane wherein it is available for convenient observation using an ophthalmoscope.
U.S. Pat. No. 4,738,521 (which is incorporated herein by way of reference) describes exemplary diagnostic ophthalmic lenses designed for use in conjunction with indirect ophthalmoscopes. This patent describes an indirect ophthalmic lens for use in examining a patient's eye comprising a homogeneous transparent optical material having two aspheric surfaces of revolution. The optical material used to form the dual aspheric lens is described as being either a glass having an index-of-refraction of 1.523 or, alternatively, an ophthalmic plastic having an index-of-refraction of 1.498. The attributes of a lens fabricated as defined in this patent allow a generally flat indirect image of the retina-under-exam to be formed free of any significant image aberrations.
Diagnostic ophthalmic lens systems such as that described in the above patent are used for both illumination and imaging purposes (as further discussed herein). As a result, however, specular surface reflections can reduce the performance of the lens system.
Diagnostic ophthalmic lens systems also must be sterilized. For example, it is common to use diagnostic ophthalmic lenses in conjunction with an indirect ophthalmoscope within an operating room environment during surgery on the eye. Because the surgeon touches the diagnostic ophthalmic lens during the course of surgery and then goes on to perform other procedures and operations within the sterile surgical field, it is necessary for the diagnostic ophthalmic lens to be properly sterilized prior to each and every surgical use. One method of effecting adequate sterilization of medical devices is through the use of a steam autoclave. In a steam autoclave, vapor under high temperature and pressure is used to effectively sterilize the surfaces of the item. However, the harsh environment of a steam autoclave will often have a deleterious effect on the optical and structural properties of standard optical glasses.